Beam control system for pickup tubes



June 6, 1961 J. HUDGINS BEAM CONTROL SYSTEM FOR PICKUP TUBES 5Sheets-Sheet 1 Filed Aug. 29, 1958 FIG. 1..

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A DIFFERENT [T LE VE CATHODE OR BEAM CURRENT MENTOR JOHN I HUDGINSATTORNEY June 6, 1961 J. l. HUDGINS 2,987,645

BEAM CONTROL SYSTEM FOR PICKUP TUBES Filed Aug. 29, 1958 5 Sheet-Sheet sPHOTOCATHODE ILLUMINATION F OT CANDLES 10 TARGET TO CATHODE POTENTIAL (EF 1 G 1B F I. G 5

TO VIDEO AMP LIFIERS LONG TIME CONSTANT VOLTAGE DOUBLING 2o RECTIFIER GRD FE EDBACK 22 CONTROL sfiBR'r INTEGRATION NETWORK INVENTOR COARSE BEAMADJUSTMENT JOHN I. HUDGINS ATTORNEY J1me 1961 J. l. HUDGINSO 2,987,6 5

BEAM CONTROL SYSTEM FOR PICKUP TUBES Filed Aug. 29, 1958 5 Sheets-Sheet5 v mikgiffi 25 RECTIFIER TARGET POTENTIAL PRESET TO VIDEO AMPLIFIERS voLTAGE DOUBLING 2o RECTIFIER 4| n AcK I ll CONTROL 2].

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l8 l TO VIDEO AMPHFIER AMPLIFIERS 1 xwfia T11?! GRID REC ER 1 FEDBACKCONTROL, 2 1' COARSE BEAM ADJUST. VOLTAGE DOUBLING 25 CATHODIEZRBCTIFILR FEEDBACK 2 4: CONTROL INVENTOR JOHN 1. HUDGINS NETWORKATTORNEY 2,987,645 BEAM CONTROL SYSTEM FOR PICKUP TUBES John I. Hudgins,Baltimore, Md., assignor to The Bendix Corporation, a corporation ofDelaware Filed Aug. 29, 1958, Ser. No. 758,127 9 Claims. (Cl. 315-11)This invention has for its primary object to provide an improved systemfor automatically controlling the scanning beam of a television pickuptube in a manner such as to provide maximum video signal output for alllight levels. The system has been successfully utilized with a pickuptube of the two-sided target, return beam type such as the imageorthicon, and accordingly is herein illustrated and described inconnection with such type of tube.

The operation of tubes of the orthicon type is now well known throughoutthe industry. In brief, a charge pattern is formed on the photocathodeside of a target which produces a similar potential pattern on theopposite or scanned side of the target. The scanning beam is atsubstantially zero axial velocity when it reaches the target, andtheoretically only sufficient electrons should land as will dischargethe elementary capacitances of the target, causing the target to returnto cathode potential (substantially cancel the charge or bleach thetarget), the remaining electrons of the beam returning to a bank ofmultipliers to produce an output signal proportional to the electronslanding on the target. By bleach, as used herein, is meant substantiallycomplete or optimum target discharge. The beam should have sufificientintensity, or should be sufficiently dense, to furnish enough electronsto carry out the discharge function and no more. Increasing theintensity of the beam beyond this point does not increase the signalbecause the number of electrons landing on the target should ideallyremain the same; it actually reduces the signal-to-noise ratio since thenumber of returning electrons are then out of propor tion to the numberwhich land, and the noise in the output signal of the tube isproportional to the square root of the beam current. Thus, unless thecharge on a target proportional to the highest level of illumination ofthe photocathode is fully cancelled for each scanning cycle, theinformation of the photocathode is not properly transferred to thereturn beam; in the case of inadequate discharge the areas which are notbleached (dark areas) will transfer little or no information, and in thecase of overblcach, the signal-to-noise ratio will be reduced. Manualcontrol of beam density is satisfactory where the light level remainssubstantially constant as in studio television, but when the light levelis constantly varying, as where an unattended pickup tube is located ata point remote from a monitoring or control station, automatic controlbecomes imperative. A certain amount of control may be had under varyinglight level conditions by compensating for changes in light level from agiven best operation value, as by the use of a light compensating irisdiaphgram or a polarized light filter. Such type of regulation, however,is only effective insofar as excessive light is concerned; it affords nohelp when the light level drops below the best operation value, at whichtime the signal-to-noise ratio should be maintained at a maximum.

Various systems or circuitry have heretofore been proposed for automaticbeam regulation to correlate the sensitivity of the pickup tube to thedegree of illumination of the area being televised. One example is wherethe pulsating signal output of the tube is amplified and then developedacross a potentiometer resistance, from which a portion is fed back andadded to the grid potential of the tube via a rectifying and integrationnet- Work in an efiort to render the beam adequate to provide 2,987,645Patented June 6, 1961 target bleach under low light level conditions.Such systems are based on the premise that bleach occurs at maximumvideo output; they are maximum video seekers in that, if bleach andmaximum video output occur at the same beam density, an increasing beamat a density less than that necessary for target bleach will produce anincreasing signal on the orthicon grid. This continues to increase thedensity of the beam until bleach occurs, whereupon the increasing beamserves to decrease signal output, reducing grid potential, and as aconsequence decreasing beam density. It has been found that such type ofcircuitry will not operate satisfactorily with a return beam type ofpickup tube. This failure is due to the fact that there has to be acertain target-tocathode potential (E at each light level for maximumvideo signal output and optimum target discharge to occur at the samebeam density. Prior known circuitry of this general type does notutilize a target potential which is a function of the level ofillumination on the photocathode of a pickup tube.

A more specific object of the present invention, therefore, is toprovide in conjunction with a television pickup tube, automatic controlcircuitry which will select and hold a value of target-to-cathodepotential such as will cause target bleach and maximum video output tooccur at the same beam density.

Another object is to provide improved beam control circuitryparticularly adapted for a pickup tube of the orthicon or return beamtype.

In carrying out the objects of the invention, I select a value oftarget-to-cathode potential (E such as will cause target bleach andmaximum video output to occur at the same beam density, and means areprovided for automatically maintaining the E at nearly optimum value. Bytarget-to-cathode potential is meant the potential difference whichexists between the target (indicated at 11 in the drawings) and theelectron gun cathode (indicated at 16). In one type of circuitry, thetarget-tocathode potential is maintained at the optimum value as afunction of the level of illumination of the photocathode, while inother forms of the invention it is so maintained as a function of thevideo signal, and in still another form by a suitable load resistor inthe cathode circuitry. The scanning beam electron density is then causedto vary as a function of the video output signal, to thereby producetarget bleach or optimum target discharge.

In the drawings:

FIG. 1 shows a family of curves, each curve plotting integrated videooutput against cathode or beam current at a fixed light level on thesurface of the photocathode of an orthicon pickup tube, the heavy dotson these lines indicating the occurrence of target bleach;

FIG. 1A is a curve chart illustrating how actual video output variesfrom the desired optimum as a result of not providing the proper E(target-to-cathode potential) for each given incident light level on thephotocathode. In this chart, the heavy dash line connecting the bleachdots represents the locus of video output when operating at a singlevalue (unchanging) E and the light-line curves plot integrated videooutput against cathode cur rent, each curve being plotted at a differentlight level;

FIG. 1B is another curve chart in which photocathode illumination isplotted against E showing the results of experiments with two differenttypes of orthicon pickup tubes;

FIG. 2 is a schematic of one type of circuitry for carrying out theobjects of the instant invention; and

FIGS. 3, 4 and 5 are additional schematic diagrams illustrating othertypes of circuitry capable of regulating the density of the scanningbeam as a function of the level of illumination of the photocathode.

It has been found that when certain elements of a pickup tube of theorthicon type are operating at given relative potentials, target bleachand maximum output signal occur simultaneously. By providing suitablecircuitry for utilizing this coincidence, the density of the scanningbeam may be automatically controlled to improve tube sensitivity and thesignal-to-noise ratio at all light levels.

FIG. 2 is an example of a circuit which will maintain the Esubstantially at optimum value, and will also maintain the density ofthe scanning beam substantially proportional to incident photocathodeillumination at all light levels. In this figure direct current flows tothe photocathode through a resistance 12, any changes in flow resultingfrom variations in photocathode light level producing a voltage dropacross resistance 12 proportional to such changes. Since these changesmay include changes of exceedingly limited magnitude, and since it isdesired to convert all changes into an amplified target feedbackpotential having an average D.C. level, the voltage or potential dropsacross resistor 12 are fed into an A.C. modulator 13, Where theymodulate a carrier Wave, which is amplified to the desired magnitude at13' and then demodulated at 14. The amplifier '13 is designed to producean output which is the inverse of its input (i.e. E Kl/E The need forthis is illustrated in FIG. 1B, where the linear plot of these functionswill show that target-to-cathode potential varies inversely as the levelof illumination. The clamping network 14' holds the amplitude of theamplified sine wave to the desired reference level of potential fortarget feedback. The potentiometer resistance 15 constitutes anadjustable feedback control and hence serves as a means for setting thetarget-to-cathode potential at a value such that target bleach andmaximum video signal output occur simultaneously, as will be moreclearly described in connection with the curve charts of FIGS. 1 and 1A.The potential difference (E between the target 11 and cathode 16 is thusautomatically varied inversely with variations in the level ofillumination of the photocathode. Feedback polarity is arrangednegative-going on' the target for an increasing photocathode current.

The video output signal is taken from the final dynode 17 of theelectron multipliers, amplified at 18 and a portion of the amplifiedvideo fed back to the control grid 19 of the pickup tube across apotentiometer resistance 20, voltage doubling rectifier 21, and anintegrating network 22. Tube 21' clamps the feedback voltage to thedesired level. By using a voltage doubling rectifier, feedback becomesproportional to peak video output, a feature common to all circuitryillustrated herein.

When the output signal increases in response to an increase inphotocathode illumination, the bias on the control grid 19 will bereduced and the density of the beam increased, and this increase willcontinue until bleach occurs. For a beam current slightly greater thanthat required for optimum target discharge, signal output is decreasedand the bias on the control grid is increased, thereby reducing beamdensity. Thus by combining the target feedback and grid feedbackcircuitry in the manner illustrated, thetarget-tocathode potential andbeam density are both caused to vary as a function of the photocathodeillumination, to in turn provide maximum video signal output for alllight levels.

The curve charts of FIGS. 1, 1A and 1B illustrate the theory ofoperation on which the invention is based. FIG. 1 shows a family ofcurves plotting average video signal output against cathode current at afixed incident light level on the surface of the photocathode 10, eachcurve representing a different value of E It will be noted that there isan optimum E (so labeled) where target bleach occurs at maximum videooutput, which is maximum under any condition for a given incident lightlevel. The proper value of E for the particular light level in FIG. 1 isat the top of curve F. In curves A to E, inclusive, overbleach willoccur, since E is less than optimum and video output remains atsubstantially the same level over a given range following bleach. Herethe signal to-noise ratio will deteriorate since the number of electronsin the return beam will be out of proportion to those that land on thetarget. In curves G, H and'I, E is greater than optimum and bleachoccurs following maximum video signal output and here, again, theelectrons of the return beam will be out of proportion to those landingon the target.

In FIG. 1A the thin full lines plot average video signal output againstcathode current with E held constant,

each line representing a different light level.

FIGURE 3 If reference is had to FIG. 1, it will be noted that within theregion prior to target bleach at any given value of beam current beyondthreshold, the video output becomes peaked concurrently with proper ESuch would be the case if from a stable condition of photocathodeillumination, target and grid potential settings, the level ofillumination then increased. 7

FIG. 3 illustrates a circuit in which video output is fed back to' thetarget as a DC. potential, causing the target to assume the correctpotential for each level of photocathode illumination; and this feedbackis regenerative prior to maximum output and degenerative beyond maximumoutput, and as such is a maximum video seeker.

In FIG. 3, the basic beam-control feedback circuit is similar to that ofFIG. 2 and other maximum video seeking types of control circuits foundin the prior art. However, instead of feeding the photocathode currentback to the target as in FIG. 2, a predetermined portion of the videooutput is amplified and then fed back to the target across a regulatingpotentiometer 23, voltage doubling rectifier 24 and integration network25. Assuming operation at some light level under conditions of properbeam density and E for optimum bleach, upon reducing light level, the E'is initially too low in value and the target is then beyond bleach asillustrated at the point X on curve A, for example. Thus in FIG. 1 itwill'be seen that if E responded at the same rate or faster than thebeam current responded, one could conceivably end up in a stablecondition at the second hump X of the curve I, which occurs when E isgreater than optimum. Therefore it becomes necessary for the'beam torespond to a change in the video signal at a rate faster thanthe targetresponds to such change (follow curve A to the vicinity of its bleachpoint prior to changing E so that the target can change 'in the nearbleach region where optimum E is a function of maximum video output,

FIGURE 4 Due to the low transconductance exhibited by the orthicon typeof pickup tube, it becomes feasible to utilize the cathode as the pointof reception of the governing potential instead of the target as inFIGS. 2 and 3. A

FIGURE 5 A simplification of the circuit of FIG. 4 can be achieved byapplying the basic beam-controlling feedback to the grid of the electrongun section of the pickup tube and causing variations in the beamcurrent to develop a voltage in the cathode circuit which willapproximate the change in E potential met when small changes in lightlevel are encountered. FIG. 5 illustrates a circuit of this gen eraltype. In this instance, the basic feedback circuit applies a portion ofthe video output to the grid 19 across the voltage doubling rectifierand integration network heretofore described in connection with FIGS. 2,3 and 4 to provide the necessary beam control. The cathode circuit hastherein a resistance-capacitance network 26, 27. The resistance 26 maybe, for example, between 5 and megohms. Whenever the beam currentincreases or decreases, there is a corresponding change in the voltagedrop across the resistance 26, which varies the target-tocathodepotential as a function of the level of illumination on thephotocathode. As in the previously described circuits, beam currentresponse should be faster than targetto-cathode potential response.Hence the R-C time constant of the cathode circuit should be relativelylong with respect to that of the grid circuit. By making the cathoderesistor a non-linear current sensitive (or voltage sensitive) type, therange of operation and stability of the circuit can be improved.

Once the basic concept of the invention has been made known to thoseskilled in the art, various types of circuitry for accomplishing thedesired result other than the circuits of FIGS. 2 to 5, inclusive, willbecome apparent. Hence the inclusion of these circuits should beinterpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a cathode ray beam video pickup tube system ineluding an evacuatedenvelope having therein a photocathode, a target and an electron beamgun provided with a cathode and beam control means; means for selectingand automatically maintaining a value of target-to-cathode potential foreach photocathode light level such that maximum video signal output andsubstantially complete target discharge or bleach occur at the same beamelectron density, and means for feeding a selected value of the videosignal output back to said beam control means.

2. In a cathode ray beam video pickup tube system including an evacuatedenvelope having therein a photocathode, a target and an electron gunprovided with a cathode and beam control means, a video output dynode,and a source of potential for said photocathode; a target feedbackcircuit adapted to impress a potential on said target varying inverselywith variations in the response of said photocathode to variations inlight level, means for adjusting the value of said feedback potential toselect a target-to-cathode potential of a value such that maximum videosignal output and substantially complete target discharge or bleachoccur at the same beam density for all photocathode light levels, and anautomatic feedback control circuit connecting said beam control meanswith the video output dynode.

3. In a cathode ray beam pickup tube system including an evacuatedenvelope having therein a photocathode, a target and an electron gunprovided with a cathode and beam control means, a video output dynode,and a source of potential for said photocathode; an automatic targetfeedback control circuit having its output connected to said target andits input connected to a source of potential varying with variations inphotocathode potential, means in said feedback circuit for convertingvariations in the input potential to a reference level of targetfeedback potential, means for setting the target-to-cathode potential ata value such that maximum video signal output and substantially completetarget discharge or bleach occur at the same beam density for all lightlevels, and another automatic feedback control circuit interconnectingsaid beam control means with the video output dynode.

4. In a cathode ray beam pickup tube system including an evacuatedenvelope having therein a photocathode, a target and an electron gunprovided with a cathode and beam control means, a video output dynode,and a source of potential for said photocathode; a supply circuitconnecting the photocathode with its source of potential and havingtherein means for creating a potential drop proportional to variationsin photocathode illumination, a target feedback circuit having meanstherein for converting said varying potential to a target referencevoltage, a target feedback control for setting the target-to-cathodepotential at a value such that maximum video signal output andsubstantially complete target discharge or bleach occur at the same beamdensity for all light levels, and an automatic feedback control circuitinterconnecting said beam control means with the video output dynode.

5. In a cathode ray beam pickup tube system of the return beam typeincluding an exacuated envelope having a photocathode, a target and anelectron gun provided with a cathode and beam control means, a videosignal output dynode, and a source of potential for said photocathode; atarget feedback circuit having its output connected to said target andits input connected to the video signal output dynode, amplifying andintegrating means in said feedback circuit for converting the pulsatingvideo output into a steady target potential of the desired magnitude,means for setting the target feedback voltage at a value such thatmaximum video signal output and substantially complete discharge orbleach occur at the same beam density, and another feedback circuitinterconnecting said beam control means and said output dynode.

6. In a system according to claim 5 wherein the time constant of saidtarget feedback circuit is relatively long with respect to that of thebeam control feedback circuit to cause the beam to respond to a changein video signal at a rate faster than the target responds to suchchanges.

7. In a cathode ray beam pickup tube system of the return beam typeincluding an evacuated envelope having therein a photocathode, a target,an electron gun provided with a cathode and a control electrode, a videooutput dynode and a source of potential for said photocathode; anautomatic feedback control circuit for establishing and maintaining aselected target-to-cathode potential having its input connected to saiddynode and its output connected to said control electrode, amplifyingand integrating network in said circuit for converting the pulsatingvideo output to a steady target potential of the desired magnitude,means for setting the target feedback voltage at a value such thatmaximum video signal output and substantially complete discharge orbleach occur at the same beam electron density, and another automaticgain control feedback circuit interconnecting said dynode and cathode.

8. In a system according to claim 7 wherein the time constant of saidcontrol electrode feedback circuit is short compared to said cathodefeedback circuit to cause the beam to respond to changes in video signalat a rate faster than the target responds to such changes.

9. In a cathode ray beam pickup tube system including an evacuatedenvelope having therein a photocathode, a target and an electron gunprovided with a cathode and control electrode, a video output dynode anda source of potential for said photocathode and target; an automaticfeedback control circuit for establishing and maintaining a selectedtarget-to-cathode potential, said circuit having its input connected tosaid dynode and its output connected to said control electrode,amplifying and integrating network in said circuit operative to convertthe pulsating video output to a steady'target potential of the requiredmagnitude, and a cathode supply circuit provided with an R-C networkhaving a long time constant relatively to that of the control electrodefeedback circuit to render beam current response faster thantarget-to-cathode potential response.

Thalner Oct. 19, 1948 Kell M31. 1, 1949

